1. Field of the Invention
The present invention relates to the coating of high temperature superalloys, such as high nickel- and high cobalt-superalloys to provide them with a protective outer layer which has improved resistance to oxidation and corrosion when subjected to such atmospheres at high temperatures. An important use of such superalloys is for turbine blades in jet aircraft or power generation engines which perform at high temperatures and in corrosive and oxidizing atmospheres.
2. Discussion of the Prior Art
It is known to form protective coatings on the surface of metal superalloy components, such as turbine blades, using metals to form layers which are more resistant to corrosion and/or oxidation at high temperatures than is the base superalloy.
According to one such procedure, disclosed in U.S. Pat. No. 3,677,789 by Bungardt et al., the base superalloy is first coated with a thin layer of noble metal, such as platinum, and is then subjected to a high temperature aluminum diffusion treatment for several hours to form a protective layer diffused into and integrated with the base superalloy. The formed protective surface layer comprises platinum aluminide which has the disadvantages of being brittle, subject to craze cracking and has low impact strength.
Diffusion coating compositions and procedures for diffusing both aluminum and chromium into superalloy base metal components in a single step are known from U.S. Pat. No. 4,293,338 by Rose et al. Thus, the prepared superalloy base component is packed into a conventional diffusion-coating container together with a powdered cementation pack coating composition containing intermetallic CO.sub.2 Al.sub.9 powder and chromium metal powder, heated to about 1925.degree.-1975.degree. F. for about one-two hours in an inert gas atmosphere, removed and post-treated in a hydrogen atmosphere for about one hour at about 1950.degree. F. A codeposited diffusion layer of aluminum and chromium is thereby provided at the superalloy metal surface, but in the absence of any platinum group metal.
According to another known procedure, disclosed in U.S. Pat. No. 4,526,814 by Shankar et al., protective diffusion layers of a platinum group metal, chromium and aluminum are formed at the surface of superalloy base components in a multi-step process in which the superalloy base component is first coated with the platinum group metal, post-platinized at about 1900.degree. F. for three hours to diffuse the platinum metal into the superalloy, then high temperature-chromized at 1950.degree. F. for eight hours to form a diffusion layer of the platinum group metal and chromium into the superalloy, and then high temperature-aluminized at about 1400.degree. F. for five hours, to form a diffusion layer of the platinum group metal, chromium and aluminum into the superalloy base surface. Such procedure is tedious and expensive because of the several steps including the post-platinizing heating step and the pre-aluminizing heating step. Also, in cases where low amounts of chromium are diffused into the protective layer the layer is limited in effectiveness of protection to high temperature oxidation and high temperature hot corrosion application (2000.degree. F.-1700.degree. F.)